In a modern steam powered electric generating plant, there exist literally miles of piping for transporting water and steam, all of which piping may in its lifetime experience a wide range of temperatures. The portions of piping in such plants which carry either steam or water at extremely high temperatures and internal pressures are most preferably fabricated from materials which are commonly known in the metallurgical industries as "austenitic" stainless steels. Such steels are well suited to the intended purposes, but are generally more expensive than other types of steel. Consequently, in other portions of piping in a steam powered electric generating plant, less expensive steels known as "low-alloy" steels are used to carry water or steam having more moderate temperatures and pressures. Thus, in a typical power plant, there will occur a number of locations where the austenitic alloy steel pipe portions are joined to the low alloy steel pipe portions.
By definition, steel is an iron-base alloy containing manganese, usually carbon, and often other alloying elements. Although many types of steel may be referred to as "stainless", as used herein, "austenitic" refers to the nonhardenable, nonmagnetic type of stainless steel. As further used herein, "low alloy" refers to those steels containing not more than 5 percent total alloying elements such as chromium (Cr) and molybdenum (Mo).
Because of their thermal and mechanical environments, these joints between low alloy steel pipes and austenitic alloy steel pipes must be able to withstand high temperatures, high pressures, thermal shock, and cyclic temperature and load applications at least as well as would the individual austenitic alloy steel pipes or the individual alloy steel pipes. In steam powered electric generating plants the combined action of high operating pressures and temperatures, and the cyclic nature of these factors increases the stress on both pipes and joints between pipes. Additionally, in order to gain operating efficiency, steam powered generating plants for the last several decades have been operated at increasingly higher temperatures and pressures thereby increasing the already stressful nature of the environment in which both pipes and joints between pipes must function.
Because different materials, and in particular different metals, expand at different rates under temperature differentials, a joint between an austenitic stainless steel pipe and a low alloy steel pipe will be under particular stress where it serves as the junction between two metals which are expanding and contracting at different rates.
In addition to the temperature-induced physical stress placed upon such a welded joint, there also exist other metallurgical or chemical problems that arise when two different metals are welded to one another. Several problems are particularly troublesome: oxide notching, carbon migration and creep.
Oxide notching represents oxidation of the low alloy steel which occurs primarily at the exterior surface of the pipe joint, for example, and travels primarily along the boundary between the dissimilar metals, thereby weakening the mechanical integrity of the joint. Although specific mechanisms are not completely understood, some opinions hold that oxide notching results from electrochemical corrosion.
Carbon migration refers to a phenomenon observed whenever elevated temperatures are experienced during operation. Under such conditions, carbon has been observed to migrate from the low alloy steel to the austenitic stainless steel weld filler material or nickel-based weld filler material, whichever is being utilized. The carbon particularly tends to increase in concentration in the weld metal proximate to the weld fusion line between the two materials. The concentrated carbon compounds form at the weld interface or in the low alloy steel providing nucleation sites for high temperature failure of the weld by creep mechanisms.
"Creep" is an atomic-scale dislocation movement of alloy materials. Creep occurs at high temperatures and initially appears in the form of cavities which form at the carbide-metal interfaces and which can reduce the structural integrity of the joint.
At some point during its service lifetime, the extent of either oxide notching or creep-related cracking will dictate that the joint will have to be replaced.
In replacing such joints, a number of attempts have been made to overcome the inherent problems in welding austenitic stainless steel to low alloy steel. Generally speaking, these attempts are all aimed at using a weld material between the low alloy steel and the austenitic stainless steel which has a coefficient of thermal expansion intermediate those of the two types of steel. The object of using such a material is to buffer the differences in thermal expansion. In addition to choosing a single weld material having these characteristics, other attempts have used a series of materials or series of pipe sections of different alloys, all aimed at minimizing the differences in thermal expansion between any two adjacent portions of pipe or of the joint itself.
One method of providing such a chemical and thermal transition between austenitic stainless steels and low alloy steels has comprised the use of a welded joint having more than one portion. A first portion, sometimes known as a "butter joint", is fused to the low alloy steel portion. The butter joint is then welded to the austenitic stainless steel portion using a weld material having characteristics between those of the austenitic stainless steel and the low alloy steel. As stated earlier, such a technique attempts to stabilize the joint by providing smaller differences in chemical potential and thermal expansion between each portion of the pipes and of the joint than would exist if the two pipes were simply welded directly one to the other.
In practice these attempts have proven somewhat undesirable. Furthermore, in recent years, the difficulties associated with the joining of dissimilar metals has been compounded in steam powered generating plants by the discovery that the original welded joints between low alloy steel pipes and austenitic stainless steel pipes in such plants are subject to an unexpectedly--and unacceptable--high rate of failure and a consequent shorter-than-expected service lifetime. In particular, welds originally believed to have 40-year service lifetimes have been failing in as few as 10 years. This difficulty is straightforward in concept but overwhelming in its repair implications: i.e. the physical difficulty of performing the necessary critical welding techniques in the environments within a steam powered electrical generating plant in which certain welded joints are found. As set out above, there are literally miles of such pipes and hundreds of such joints in such plants and a tremendous number of these joints are found within the plant in locations to which access is extremely difficult. The difficulty in reaching such welds compounds the difficulty and geometrically increases the expense of replacing them as such welds must be made by highly skilled welders and in many applications, including nuclear power plants, must be X-rayed before operation can be recommenced.
There thus exists the need for a method of forming welded joints between low alloy steels and austenitic stainless steels which method provides a joint which retains its integrity in the face of extreme changes in temperature, mechanical stress and chemical corrosion. Finally, such a method should provide for the welds to be made and installed in the most economical and serviceable manner.
It is an object of this invention to provide a stable welded joint between dissimilar metals which joint is resistant to chemical corrosion and mechanical stress, has an extended service lifetime, and which can be installed in otherwise-difficult environments at a minimum of expense and logistical difficulty.